The present invention relates to a gas sensor having a sensor element exposed to a gas under measurement so as to detect a specific gas component in the gas under measurement and, more particularly, to a gas sensor having a sensor element formed with a porous protection layer.
Hereinafter, the term “front” refers to a gas sensing side with respect to the direction of an axis of a gas sensor; and the term “rear” refers to a side opposite the front side.
Conventionally known is a gas sensor of the type mounted for use on an exhaust pipe of an internal combustion engine etc. and equipped with a sensor element to generate an electromotive force or electrical resistance according to the concentration of a specific gas component such as NOx (nitrogen oxide) or oxygen in an exhaust gas of the internal combustion engine. The sensor element has a detection portion formed on a front end part thereof and adapted to, when heated to a predetermined high temperature by a heater etc., detect the specific gas component. Under such a high-temperature state, however, there is a fear of breakage (e.g. cracking) of the sensor element due to thermal impact at the time when water drops contained in the exhaust gas adhere to the detection portion of the sensor element (i.e. the detection portion of the sensor element gets wet with water). A gas sensor is thus developed in which a sensor element has a detection portion covered with a porous protection layer and protected from water as disclosed in Japanese Laid-Open Patent Publication No. 2009-115781.
More specifically, Japanese Laid-Open Patent Publication No. 2009-115781 discloses a gas sensor that includes a metal shell (metal housing) 1100, a ceramic holder 300 formed of an insulating ceramic material (such as alumina) and placed in an axial inner hole 1100h of the metal shell 1100 and a sensor element 21 held in an insertion hole 320 of the ceramic holder 300 as shown in FIG. 8. The sensor element 21 has, at a front end part thereof, a detection portion covered with a protection layer 25. The detecting portion with the protection layer 25 protrudes toward the front from the ceramic holder 31 such that a rear end part 26 of the protection layer 25 is accommodated within the axial inner hole 1100h of the metal shell 1100. Further, metallic protector members (protection tubes) 510 and 610 are attached to a front end portion of the metal shell 110 so as to protect therein the sensor element 21. In order to prevent the protection layer 25 from being damaged by collision with the ceramic holder 300 during insertion of the sensor element 21 into the insertion hole 320 of the ceramic holder 300, the rear end part 26 of the protection layer 25 is situated in front of and spaced apart from a front end of the insertion hole 320 of the ceramic holder 300. Thus, a side surface P of the sensor element 21 between the insertion hole 320 of the ceramic holder 300 and the rear end part 26 of the protection layer 25 is left uncovered and unprotected by the protection layer 25.